A cold cathode display device is a display device which causes electrons emitted from an electron emitting part thereof to collide with a phosphor in a space formed by disposing a pair of substrates, at least one of which is transparent, opposite to each other, thereby displaying a desired pattern. FIG. 13 illustrates a structure of a conventional cold cathode display device.
A back substrate 101 and a face substrate 102 are disposed opposite to each other with a spacer 103 interposed therebetween, to form a chamber. The chamber is evacuated. Each of the back substrate 101 and the face substrate 102 is attached to the spacer 103 by glass frit 104, and at least a portion of the face substrate 102 which serves as a display surface is required to be transparent in view of properties of a cold cathode display device. A light emitting part is formed on an inner side of the face substrate 102 in order to display a desired pattern. The light emitting part is formed by depositing a phosphor 108 on a transparent electrode 109 serving as a positive electrode (, which part will hereinafter be also referred to as an “anode”).
On the other hand, an electron emitting part is formed on an inner side of the back substrate 101, so as to be opposite to the anode. The electron emitting part is formed by depositing a cold cathode material 106 on a substrate electrode 105 serving as a negative electrode (, which part will hereinafter be also referred to as a “cathode”). While a filament has conventionally been employed as such an electron emitting part, a conductive layer including a carbon nanotube which can be manufactured by a printing process has become used as a material for a field emission type cold cathode, recently. Reasons for recent use of a conductive layer including a carbon nanotube as an electron source are higher brightness and a longer life time as compared to those provided by use of a filament. Also, as a conductive layer can be manufactured by a printing process, low cost manufacture is possible. Meanwhile, details of a technique for employing a conductive layer including a carbon nanotube as a material for a field emission type cold cathode are provided in Japanese Patent Application Laid-Open No. 2001-155666.
Further, an extraction electrode 107 for controlling electrons is provided between the anode and the cathode. The extraction electrode 107 has many apertures through which electrons emitted from the cathode pass, the apertures being located at positions at which the extraction electrode 107 and the cathode intersect each other. The extraction electrode 107 is configured such that a leg portion, formed by bendin6fritg a portion of the extraction electrode 107, is attached to the back substrate 101 by glass frit, and is secured to the back substrate 101. There is a need of externally supplying a potential to the extraction electrode 107. For this reason, the extraction electrode 107 is connected to a copper wire electrode 110, a portion of which penetrates the glass frit 104 to protrude from the chamber, within the chamber. As there is a need of externally supplying a potential also to each of the substrate electrode 105 and the transparent electrode 109, each of the substrate electrode 105 and the transparent electrode 109 is connected to the copper wire electrode in an analogous manner to the extraction electrode 107. It should be noted that FIG. 13 illustrates only connection between the extraction electrode 107 and the copper wire electrode 110.
Next, principles of operations of the cold cathode display device will be explained. Basically, operations of the cold cathode display device are similar to those of a triode. Upon application of a potential to the substrate electrode 105 of the cathode within the chamber holding therein a vacuum with a pressure in a range between approximately 10□3 and 10□5 Pa, electrons are emitted from the cold cathode material 106. The emitted electrons are controlled by the extraction electrode 107, and are accelerated because of a potential difference between the transparent electrode 109 of the anode and the substrate electrode 105 of the cathode. The accelerated electrons reach the phosphor 108 of the anode, and excite the phosphor. The excited phosphor emits light when returning to a normal energy state. The cold cathode display device provides a desired display by utilizing the light emission of the phosphor.
The conventional cold cathode display device is a simple triode which is composed of an anode, an extraction electrode and a cathode. With this composition, the following problems have been caused.
A structure of the extraction electrode of the conventional cold cathode display device has been designed to have an optimum diameter of the aperture in the extraction electrode, an optimum plate thickness of the extraction electrode and an optimum distance between the extraction electrode and the cathode, taking into account mainly an extraction voltage and an extraction efficiency. However, optimization of a diameter of the aperture in the extraction electrode, a plate thickness of the extraction electrode and a distance between the extraction electrode and the cathode could not allow sufficient reduction of a size of electrons (which will hereinafter be also referred to as an “electron beam”) emitted from the cathode, which is measured on a surface of the anode. As such, a distance which the electron beam travels until it reaches the surface of the anode should be reduced, thereby making the size of the electron beam as measured on the surface of the anode (which will hereinafter be also referred to an “electron beam diameter”) smaller than a size of the phosphor of the anode. This requires a distance between the anode and the extraction electrode to be reduced.
Due to the requirement that the distance between the anode and the extraction electrode be reduced, a voltage which can be applied between the anode and the extraction electrode is limited, so that a high voltage can not be applied. Being unable to apply a high voltage to the anode results in a failure to sufficiently enhance an efficiency in light emission of the phosphor. This causes a problem of non-achievement of a cold cathode display device providing a satisfactory brightness.
The requirement that the distance between the anode and the extraction electrode be reduced, on the other hand, results in reduction of a distance between the cathode and the anode. Accordingly, there is a need for configuring the cold cathode display device to have a ratio of approximately 1:1 between a size of the electron emitting part of the cathode and a size of the phosphor of the anode. As a result, in a situation where a voltage on the extraction electrode is varied in order to adjust a current value so that a degree of convergence in the vicinity of the extraction electrode is varied to further vary an electron beam diameter, the variation in electron beam diameter directly affects light emission of the phosphor of the anode, resulting in variation in brightness among pixels.
Moreover, the requirement that the distance between the anode and the extraction electrode be reduced makes a required level of an accuracy in assembling, high. A low accuracy in assembling results in positional shift of an electron beam, to bring about emission of mixed colors in which another phosphor located next to an intended phosphor emits light. This causes a problem of degradation in color purity.
A further problem of localization of electrons in emission thereof from a surface of the cathode is caused. Causes of this problem are as follows. In a typical cold cathode electron source, emission characteristic thereof is determined by a strength of an electric field and a work function of an uneven surface of a cathode with protrusions. However, an electric field strength is very responsive to respective configurations of the protrusions. Even if a work function of the surface of the cathode can be made uniform in some way, it is technically difficult to planarize the surface of the cathode with an accuracy on the order of μm or smaller. Accordingly, variation in height among the protrusion of the surface of the cathode is unavoidable, to allow an amount of electrons emitted from the cathode to depend greatly on an electric field of the surface of the cathode. Hence, there are created a portion which can easily emit electrons and a portion which can not easily emit electrons due to subtle variation in configuration among the protrusions in the surface of the cathode. In the portion which can easily emit electrons, a current value increases exponentially in accordance with an increase of the electric field of the surface after electron emission is initiated. As a result, localization of an electron emitting region occurs on the surface of the cathode so that light emitting points are interspersed like dots in a pixel which is lighted up, which causes a problem of degrading an image quality.